Liquid crystal display device and manufacturing method thereof

ABSTRACT

Disclosed is a manufacturing method of a liquid crystal display device which is a manufacturing method of a liquid crystal display device including a liquid crystal alignment film to which an alignment regulating force is imparted by a photo-alignment treatment, including: a film forming step of forming a film containing a polymer whose main chain is cleaved by irradiation with light; a photo-alignment step of imparting an alignment regulating force to the film formed in the film forming step by irradiation of the film with light in an atmosphere of a temperature lower than 100° C.; and a removing step of removing a low-molecular weight component generated by cleaving the main chain of the polymer through the light irradiation after the light irradiation. Also disclosed is a liquid crystal display device manufactured by the manufacturing method.

CROSS-REFERENCE TO RELATED APPLICATION

The present application claims priority from Japanese applicationJP2009-260076 filed on Nov. 13, 2009, the content of which is herebyincorporated by reference into this application.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a liquid crystal display device and amanufacturing method thereof.

2. Description of the Related Art

A liquid crystal alignment film to be used in a liquid crystal displaydevice is formed through an alignment treatment, i.e., a step ofimparting the liquid crystal alignment capability. As for the alignmenttreatment, the development of a photo-alignment treatment has beenadvanced recently. The photo-alignment treatment is a method ofimparting the liquid crystal alignment capability to the surface of anorganic film by irradiation of the surface of the organic film formed ona surface of a substrate with substantially linearly polarized light.

As a related art, JP 2007-226097 A discloses a manufacturing method of aliquid crystal display device using a liquid crystal display panel whichenables high-definition display by preventing a decrease in thealignment control ability. More specifically, JP 2007-226097 A disclosesa method of imparting alignment anisotropy to a film by irradiation ofthe film with polarized ultraviolet light while heating to 100° C. orhigher in a photo-alignment treatment.

Further, JP 2003-255349 A discloses a method of forming a liquid crystalalignment film having alignment anisotropy by irradiation of a polyamicacid thin film with polarized ultraviolet light, followed by conversionof polyamic acid into a polyimide.

SUMMARY OF THE INVENTION

However, a liquid crystal alignment film formed with a photo-alignmentagent or by a photo-alignment treatment disclosed in JP 2007-226097 A orJP 2003-255349 A has a problem that the alignment anisotropy is low. Thefact that the alignment anisotropy is low means that the liquid crystalalignment regulating force is weak. A liquid crystal display devicehaving a liquid crystal alignment film with a weak liquid crystalalignment regulating force has a problem that the image characteristicof a liquid crystal component cannot be sufficiently obtained.

The present inventors found that such a decrease in the alignmentregulating force is caused as follows. A polymer which photodegrades ina photo-alignment treatment produces a low-molecular weight component,which remains in a liquid crystal alignment film and causes a decreasein the alignment regulating force and also causes a decrease in theimage characteristic of a liquid crystal component.

Therefore, an object of the invention is to provide a liquid crystaldisplay device using a liquid crystal alignment film formed by aphoto-alignment treatment in which the alignment anisotropy of theliquid crystal alignment film is increased. The above and other objectsand novel characteristics of the invention will be apparent from thedescription of this specification and the accompanying drawings.

A manufacturing method of a liquid crystal display device according tothe invention is a manufacturing method of a liquid crystal displaydevice including a liquid crystal alignment film to which the alignmentregulating force is imparted by a photo-alignment treatment, andincludes: a film forming step of forming a film containing a polymerwhose main chain is cleaved by irradiation with light; a photo-alignmentstep of imparting the alignment regulating force to the film formed inthe film forming step by irradiation of the film with light in anatmosphere of a temperature lower than 100° C.; and a removing step ofremoving a low-molecular weight component generated by cleaving the mainchain of the polymer through the light irradiation after the lightirradiation.

Further, in the removing step, the low-molecular weight component may beremoved by heating the film after the light irradiation. Further, theheating in the removing step may be performed under a reduced pressureatmosphere.

Further, in the removing step, the low-molecular weight component may beremoved by washing the film after the light irradiation. Further, thewashing in the removing step may be performed using a solutioncontaining a water-soluble organic solvent. Further, the washing in theremoving step may be performed using a solution containing a surfactant.

Further, the polymer may be a compound represented by the followingchemical formula (1). In the formula (1), R represents a divalentelectron-donating organic group and n represents the number of repeatingunits of the polymer.

Further, the invention provides a liquid crystal display devicemanufactured by any of the above-mentioned respective manufacturingmethods.

In the liquid crystal display device using a liquid crystal alignmentfilm formed by a photo-alignment treatment, a decrease in the imagecharacteristic of a liquid crystal component can be reduced.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a pixel portion illustrating thestructure of a pixel of a liquid crystal display device according to anembodiment of the invention.

FIG. 2A is a plan view of a pixel portion illustrating the structure ofa pixel of a liquid crystal display device according to an embodiment ofthe invention.

FIG. 2B is a cross-sectional view of a pixel portion illustrating thestructure of a pixel of a liquid crystal display device according to anembodiment of the invention and is a view showing a cross-section alongthe line 2B-2B in FIG. 2A.

FIG. 2C is a cross-sectional view of a pixel portion illustrating thestructure of a pixel of a liquid crystal display device according to anembodiment of the invention and is a view showing a cross-section alongthe line 2C-2C in FIG. 2A.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, embodiments of the invention will be described withreference to the drawings.

A manufacturing method of a liquid crystal display device according toan embodiment of the invention is a manufacturing method of a liquidcrystal display device including a liquid crystal alignment film towhich the alignment regulating force is imparted by a photo-alignmenttreatment, and includes: a film forming step of forming a filmcontaining a polymer whose main chain is cleaved by irradiation withlight; a photo-alignment step of imparting the alignment regulatingforce to the film formed in the film forming step by irradiation of thefilm with light in an atmosphere of a temperature lower than 100° C.;and a removing step of removing a low-molecular weight componentgenerated by cleaving the main chain of the polymer through the lightirradiation after the light irradiation.

The manufacturing method of a liquid crystal display device according toan embodiment of the invention may be, for example, a manufacturingmethod of an in-plane switching (IPS) mode liquid crystal displaydevice, a vertically aligned (VA) mode liquid crystal display device, atwisted nematic (TN) mode liquid crystal display device, or a liquidcrystal display device employing other driving mode.

For example, the manufacturing method of a liquid crystal display deviceaccording to an embodiment of the invention may be a manufacturingmethod of a liquid crystal display device including a pair of substratesat least one of which is transparent, a liquid crystal layer disposedbetween the pair of substrates, a group of electrodes formed on at leastone of the pair of substrates to apply an electric field to the liquidcrystal layer, and a liquid crystal alignment film to which thealignment regulating force is imparted by a photo-alignment treatmentand which is disposed on at least one of the pair of substrates, andincludes the above-mentioned film forming step, photo-alignment step,and removing step.

First, the polymer whose main chain is cleaved by irradiation with lightaccording to the invention will be described. The polymer whose mainchain is cleaved by irradiation with light according to the invention isa polymer which directly photodegrades. That is, the polymer absorbslight irradiated from outside and the polymer itself photodegrades.

Examples of the polymer whose main chain is cleaved by irradiation withlight include polyimides. Further, among polyimides, a cyclobutanepolyimide synthesized using cyclobutane tetracarboxylic acid dianhydrideas a starting material is preferred.

Specific examples of the cyclobutane polyimide include a compoundrepresented by the following chemical formula (1). The polymer in themanufacturing method of a liquid crystal display device according to anembodiment of the invention is preferably a compound represented by thefollowing chemical formula (1).

In the formula, R represents a divalent electron-donating organic group,and n represents the number of repeating units of the polymer and ispreferably an integer of 100 to 1000.

When the compound represented by the above chemical formula (1) isirradiated with ultraviolet light, the compound represented by the abovechemical formula (1) absorbs the ultraviolet light and undergoes aphotodegradation reaction in which the ring structure of cyclobutane isopened and a maleimide end is generated.

When the photodegradation reaction of the compound represented by theabove chemical formula (1) proceeds, a photodegradation product having amolecular weight lower than the compound represented by the abovechemical formula (1), which initially has a high molecular weight, isgenerated.

Incidentally, the compound represented by the above chemical formula (1)is only an example illustrating the compound whose main chain is cleavedby irradiation with light and the polymer is not limited thereto.

Subsequently, the film forming step according to the invention will bedescribed. The film forming step can be performed, for example, using aliquid crystal alignment agent varnish obtained by dissolving at leastone compound which forms a liquid crystal alignment film in a solvent.For example, when the compound represented by the above chemical formula(1) is formed into a film, the film forming step can be performed usinga liquid crystal alignment agent varnish obtained by dissolving apolyamic acid which is a precursor of the compound represented by theabove chemical formula (1) in a solvent.

A polyimide as the compound represented by the above chemical formula(1) is insoluble in various petroleum solvents, and therefore it isdifficult to directly add a polyimide to the liquid crystal alignmentagent varnish. Therefore, a polyimide is added to the liquid crystalalignment agent varnish in the form of a polyamic acid and/or a polyamicacid ester, both of which are a polyimide precursor. That is, when aliquid crystal alignment film containing a polyimide as a main componentis formed, a polyamic acid and/or a polyamic acid ester are/is containedin the liquid crystal alignment agent varnish.

The compound is not limited to a polyimide, and when a compoundconstituting the liquid crystal alignment film is insoluble in a solventto be contained in the liquid crystal alignment agent varnish, thecompound constituting the liquid crystal alignment film in the form of aprecursor or the like may be contained in the liquid crystal alignmentagent varnish as the compound forming the liquid crystal alignment film.

Further, the solvent to be contained in the liquid crystal alignmentagent varnish can be appropriately changed according to the polymerconstituting the liquid crystal alignment film.

The applying method of the liquid crystal alignment agent varnish is notlimited to a specific method, and for example, the application can beperformed using, for example, a cylinder-type pressing printer, aninkjet applicator, or the like. After the liquid crystal alignment agentvarnish is applied to a substrate on which the liquid crystal alignmentfilm is to be formed, heat is applied from outside to evaporate thesolvent contained in the varnish, whereby the film can be formed.

After the liquid crystal alignment agent varnish is applied onto apredetermined substrate, the solvent contained in the varnish isevaporated, whereby the film can be formed.

Subsequently, the photo-alignment step according to the invention willbe described. The photo-alignment step according to the invention is aphoto-alignment step of imparting the alignment regulating force to thefilm formed in the film forming step by irradiation of the film withlight in an atmosphere of a temperature lower than 100° C.

In the photo-alignment treatment, the polymer whose main chain iscleaved by irradiation with light contained in the film is considered toundergo three reactions as follows: (1) a reaction in which the polymerconstituting the film photodegrades at a certain probability byirradiation with light; (2) a reaction in which the polymer having amolecular chain length reduced by the photodegradation is translocateddue to thermal motion and rearranged; and (3) a reaction in which therearranged polymer is crosslinked again at a certain probability.

Other than the polymer constituting the liquid crystal alignment filmhaving a molecular chain length reduced by the photodegradation, alow-molecular weight component is also generated as a byproduct. Thislow-molecular weight component is, for example, a free componentgenerated by photodegradation in the vicinity of the end of the polymerbefore irradiation with light, and does not contribute to the impartingof the alignment regulating force to the liquid crystal alignment film.

Further, the low-molecular weight component causes a decrease in thealignment regulating force of the liquid crystal alignment film. Whenthe generated low-molecular weight component is crosslinked to a sidechain of the polymer constituting the liquid crystal alignment film andremains there, it becomes a major cause of the decrease in the alignmentregulating force. Further, if the low-molecular weight component is oncecrosslinked to a side chain of the polymer constituting the liquidcrystal alignment film, it is extremely difficult to remove thecrosslinked low-molecular weight component from the liquid crystalalignment film thereafter.

In the photo-alignment treatment according to the invention, thecrosslinking reaction between the low-molecular weight component and aside chain of the polymer constituting the liquid crystal alignment filmcan be suppressed by irradiation with light in an atmosphere of atemperature lower than 100° C. That is, by irradiation with light, thegenerated low-molecular weight component remains in the liquid crystalalignment film in an uncrosslinked state.

The low-molecular weight component remaining in an uncrosslinked statealso causes a decrease in the alignment regulating force of the liquidcrystal alignment film and therefore should be removed. The step ofremoving this low-molecular weight component will be described in detaillater.

Further, it is preferred that the photo-alignment step of imparting thealignment regulating force to the film formed in the film forming stepis performed by irradiation of the film with light in an atmosphere of75° C. or lower. Further, it is more preferred that light is irradiatedin an atmosphere of 50° C. or lower, and it is particularly preferredthat light is irradiated in an atmosphere of 25° C. or lower.

Examples of the light to be used in the photo-alignment treatmentinclude ultraviolet light, infrared light, and visible light. In themanufacturing method of a liquid crystal display device according to anembodiment of the invention, it is preferred that the photo-alignmenttreatment is performed using polarized ultraviolet light obtained bysubstantially linearly polarizing ultraviolet light.

The same shall apply to the case where ultraviolet light is irradiated,and it is preferred that ultraviolet light is irradiated in anatmosphere of 75° C. or lower. Further, it is more preferred thatultraviolet light is irradiated in an atmosphere of 50° C. or lower, andit is particularly preferred that ultraviolet light is irradiated in anatmosphere of 25° C. or lower.

Subsequently, the removing step according to the invention will bedescribed. By removing the low-molecular weight component which isuncrosslinked to the liquid crystal alignment film formed through theabove-mentioned film forming step and photo-alignment step and remainson the surface of the liquid crystal alignment film, a higher alignmentregulating force is imparted to the liquid crystal alignment film.

The alignment regulating force of the liquid crystal alignment film isstrongly affected by the surface condition of the film. Therefore, it isnecessary to remove the remaining low-molecular weight component fromthe surface of the liquid crystal alignment film in the removing step.

In other words, even if the low-molecular weight component remains inthe inside of the formed liquid crystal alignment film, thelow-molecular weight component remaining in the inside of the film doesnot largely affect the alignment regulating force as compared with thelow-molecular weight component remaining on the surface. Therefore, thelow-molecular weight component may remain in the inside of the liquidcrystal alignment film even after the removing step.

Incidentally, for example, in the photo-alignment step, the lightirradiation may be performed a plurality of times. In this case, theremoving step may be performed each time after the light irradiationperformed a plurality of times, and also may be performed only after thelast light irradiation among the plurality of irradiation operations.

Further, in the removing step, it is preferred that the low-molecularweight component is removed by heating the film after the lightirradiation. As for the heating method, for example, infrared heating,hot-air heating, hot-plate heating, or a combination of any of thesemethods can be appropriately performed.

As for the heating temperature, it is preferred that the heating isperformed at a temperature not lower than the temperature at which thelow-molecular weight component remaining on the surface of the filmafter the light irradiation evaporates. Further, it is preferred thatthe heating temperature is appropriately changed according to thephysical property of the film after the light irradiation. For example,the upper limit of the heating temperature is preferably a temperaturenot higher than the softening point of the polymer which forms the filmafter the light irradiation.

Specifically, it is preferred that the low-molecular weight component isremoved by heating the film after the light irradiation to a temperaturewithin a range from 150° C. to 300° C. It is also preferred that thelow-molecular weight component is removed by heating the surface of thefilm after the light irradiation to a temperature within a range from150° C. to 300° C.

It is more preferred that the low-molecular weight component is removedby heating the surface of the film after the light irradiation to atemperature within a range from 200° C. to 290° C. in the removing step.

Further, it is preferred that the step of removing the low-molecularweight component by heating is performed for 5 minutes to 60 minutes.When the heating time is less than 5 minutes, the low-molecular weightcomponent does not sufficiently evaporate and remains on the surface ofthe liquid crystal alignment film, and when the heating time is morethan 60 minutes, the thermal deterioration of other members may becaused.

That is, in the removing step, it is particularly preferred that thelow-molecular weight component is removed by heating the surface of thefilm after the light irradiation to a temperature within a range from200° C. to 290° C. for 5 minutes to 15 minutes.

Further, it is preferred that the heating in the removing step isperformed under a reduced pressure atmosphere. By performing the heatingtreatment under a reduced pressure atmosphere, the evaporation of thelow-molecular weight component is accelerated, and therefore, the timeof the removing step is reduced and the reliability of the removal isincreased.

The “reduced pressure atmosphere” as used herein refers to an atmospherehaving a pressure lower than the normal atmospheric pressure. That is,it is preferred that the heating in the removing step is performed underan atmosphere of about 100 kPa or less (lower than the normalatmospheric pressure). Further, it is particularly preferred that theheating in the removing step is performed under a vacuum atmosphere(10⁻¹ Pa or less).

Further, in the removing step, it is preferred that the low-molecularweight component is removed by washing the film after the lightirradiation. As for the washing method, for example, washing with purewater, washing with a brush, ultrasonic washing, or a combination of anyof these methods can be appropriately performed.

Further, it is preferred that the washing in the removing step isperformed using a solution containing a water-soluble organic solvent.

Examples of the water-soluble organic solvent as used herein includealcohol compounds having a boiling point of 110° C. or lower such asmethanol, ethanol, propanol (such as isopropyl alcohol), and butanol,low-boiling ketone compounds such as acetone, and water-solubleheterocyclic compounds having a boiling point of 250° C. or lower suchas N-methylpyrrolidone and γ-butyrolactone.

The water-soluble organic solvent described above is illustrative onlyand is not limited to these compounds.

Further, it is preferred that the washing in the removing step isperformed using a solution containing a surfactant.

Examples of the surfactant include nonionic surfactants such aspolyoxyethylene alkyl ethers (such as polyoxyethylene lauryl ether,polyoxyethylene cetyl ether, and polyoxyethylene oleyl ether),polyoxyethylene alkyl aryl ethers, polyoxyethylene alkyl esters,polyoxyethylene alkylamine sorbitan fatty acid esters, andpolyoxyethylene sorbitan fatty acid esters.

The surfactant described above is illustrative only and is not limitedto these compounds.

Also from the viewpoint of handling property in the washing step, thesurfactant is preferably any of the nonionic surfactants. However, forexample, a cationic surfactant, an anionic surfactant, an amphotericsurfactant may be used within a range that does not impair the effect ofthe invention.

Further, the washing in the removing step may be performed using, forexample, an aqueous solution in which a plurality of surfactants aremixed. That is, the washing in the removing step may be performed usingan aqueous solution containing at least two or more surfactants.Further, the washing in the removing step may be performed using anaqueous solution containing a surfactant and a water-soluble organicsolvent.

Incidentally, in the removing step, the low-molecular weight componentmay be removed by placing the film after the light irradiation under areduced pressure atmosphere or vacuum (10⁻¹ Pa or lower).

The liquid crystal display device manufactured by any of themanufacturing methods described above can reduce a decrease in the imagecharacteristic of a liquid crystal component.

Hereinafter, an example in which a polyimide represented by thefollowing chemical formula (1) is used as the compound which forms theliquid crystal alignment film will be specifically described.

In the formula, R represents a divalent electron-donating organic group,and n represents the number of repeating units of the polymer and ispreferably an integer of 100 to 1000.

It is known that the energy transfer of a cyclobutane ring from anaromatic imide ring is involved in the photodegradation of a cyclobutanering which induces the alignment anisotropy of a polyimide-based liquidcrystal alignment film having been subjected to the photo-alignmenttreatment.

Incidentally, in the compound represented by the chemical formula (1),in order to further facilitate the above-mentioned energy transfer, anelectron-donating group (R) is chemically bonded in the vicinity of thearomatic imide ring.

In the compound represented by the above chemical formula (1), thephotodegradation reaction of the cyclobutane ring proceeds moresmoothly. Further, by appropriately changing the wavelength range orirradiation dose of the ultraviolet light, a preferred photodegradationreaction is achieved.

Examples of the group represented by R in the above chemical formula (1)include an amino group (—NH—) and an alkoxy group (—OC_(n)H_(2n)—).

The functional groups described above are shown only as examples of theelectron-donating group, and the electron-donating group is not limitedto these functional groups.

Also in the case where the electron-donating group (R) is not chemicallybonded in the vicinity of the aromatic imide ring of the compoundrepresented by the above chemical formula (1), the photodegradationreaction of the cyclobutane ring is effected depending on the conditionof the light irradiation or the like. That is, it is a more preferredcondition that the electron-donating group (R) is chemically bonded inthe vicinity of the aromatic imide ring of the compound represented bythe above chemical formula (1).

Further, in order to decrease the irradiation dose of ultraviolet lightrequired for alignment anisotropy (increase the sensitivity), it isnecessary to optimize the baking degree and time which determine theimidization ratio so as to increase the imidization ratio and improvethe efficiency of the above-mentioned energy transfer.

In order to form a liquid crystal alignment film having a highimidization ratio, first, it is necessary to accurately determine theconversion (imidization ratio) of a polyamic acid and/or a polyamic acidester, both of which are a polyimide precursor, into a polyimide.

The imidization ratio is determined by comparing the intensity of aspecific absorption peak (the stretching vibration of a benzene ring anda tertiary nitrogen atom: φ-N) in the IR spectrum of the baked liquidcrystal alignment film with the φ-N absorption peak intensity of aliquid crystal alignment film baked at 300° C.

The imidization reaction does not necessarily proceed to 100%, andproceeds preferably to 40% to 100%, more preferably to 50% to 95%,further more preferably to 60% to 90% of the total reaction.

As the proceeding degree of the imidization reaction is higher, thephoto-alignment property is increased and the liquid crystal alignmentstability is increased. However, if it is too high, the resistivity ofthe liquid crystal alignment film is increased, and therefore it is notpreferred from the viewpoint of an electrical characteristic.

Further, the molecular weight of the polyimide constituting an alignmentcontrol film is preferably higher, and a polyamic acid ester does notcause a decrease in the molecular weight during heating unlike apolyamic acid, and achieves high liquid crystal alignment stability, andtherefore is more preferred.

As described above, the present inventors found that the decrease in thealignment regulating force is caused as follows. A polymer whichphotodegrades in a photo-alignment treatment produces a low-molecularweight component, which remains in a liquid crystal alignment film andcauses a decrease in the alignment regulating force and also causes adecrease in the image characteristic of a liquid crystal component.

The liquid crystal display device according to an embodiment of theinvention may be, for example, an in-plane switching (IPS) mode liquidcrystal display device, a vertically aligned (VA) mode liquid crystaldisplay device, a twisted nematic (TN) mode liquid crystal displaydevice, or a liquid crystal display device employing other driving mode.

Incidentally, the remaining amount of the above-mentioned low-molecularweight component was calculated from the difference in the spectrum of aspecific absorption peak intensity between before and after apredetermined step of removing the low-molecular weight component usingFT-IR.

Further, as for an evaluation method for the alignment anisotropy of theliquid crystal alignment film, the evaluation was performed using thevalue of a polarized dichroic ratio. The polarized dichroic ratio iscalculated from the following equation (I) using an absorbance at eachwavelength obtained from the absorption spectra of both s-polarizationand p-polarization.Polarized dichroic ratio=(A _(⊥) −A _(//))/(A _(⊥) +A _(//))  (I)

In the above equation (I), A_(//) represents the absorbance of theliquid crystal alignment film in the direction parallel to the polarizedlight of the measuring light, and A_(⊥) represents the absorbance in thedirection perpendicular to the polarized light of the measuring light.In the invention, the alignment anisotropy of each liquid crystalalignment film was compared using the dichroic ratio at a wavelengthwhich gives the maximum polarized dichroic ratio.

For example, when the polyimide-based liquid crystal alignment film isirradiated with polarized ultraviolet light, the number of polyimidemolecules in the direction parallel to the electric field vector of theirradiation light is decreased and the alignment anisotropy is induced.

The absorbance A_(//) measured by the polarized light in the directionparallel to the polarized irradiation light is smaller than theabsorbance A_(⊥) measured by the polarized light in the directionperpendicular to the polarized irradiation light, and therefore, thepolarized dichroic ratio is larger than 0.

Further, A_(⊥) and A_(//) are proportional to the number of polyimidemolecules in the liquid crystal alignment film, and therefore, as thepolarized dichroic ratio of the liquid crystal alignment film isincreased, the alignment anisotropy is increased.

Further, when the alignment anisotropy is increased, there is exhibitedan effect that the intensity of an AC residual image (burn-in) can besignificantly reduced.

Further, the anchoring strength of the liquid crystal alignment film isalso an important factor required for the liquid crystal alignment film.The anchoring strength indicates the degree of binding of a liquidcrystal molecule to the surface of the liquid crystal alignment film andalso indicates the degree of resistance to a change in the azimuth andpolar angle direction with respect to a force applied from outside in aninterface between the liquid crystal molecule and the liquid crystalalignment film. An increase in the alignment anisotropy is an importantfactor for increasing the anchoring strength.

The liquid crystal display device according to an embodiment of theinvention will be described with reference to FIG. 1, FIG. 2A, FIG. 2B,and FIG. 2C.

FIG. 1 is a schematic cross-sectional view showing one pixel and thevicinity of one pixel of a liquid crystal display device according tothe present embodiment. FIG. 2A to FIG. 2C are schematic views of anactive matrix substrate illustrating the structure of one pixel and thevicinity of one pixel of a liquid crystal display device according tothe present embodiment: FIG. 2A is a plan view; FIG. 2B is across-sectional view along the line 2B-2B in FIG. 2A; and FIG. 2C is across-sectional view along the line 2C-2C in FIG. 2A. Further, FIG. 1corresponds to a portion of a cross-section along the line 2B-2B in FIG.2A.

Further, FIG. 2B and FIG. 2C are schematic views with emphasis on thestructure of a main portion and do not correspond one-to-one to thecross-sections taken along the lines 2B-2B and 2C-2C in FIG. 2A,respectively.

For example, in FIG. 2B, a semiconductor film 116 is not shown, and inFIG. 2C, among through holes 118 for connecting a common electrode 103to a common electrode line (common line) 120, only one isrepresentatively shown.

In this embodiment, a scan line (gate electrode line) 104 and a commonelectrode line 120, which are made of chromium (Cr), are disposed on aglass substrate 101 as the active matrix substrate, and a gateinsulating film 107 made of silicon nitride is formed so as to cover thescan line 104 and the common electrode line 120.

Further, on the scan line 104, a semiconductor film 116 made ofamorphous silicon or polysilicon is disposed through the gate insulatingfilm 107. This semiconductor film 116 serves as the active layer of athin film transistor (TFT) 115 as an active element.

Further, a signal line (drain electrode) 106 made of Cr—Mo(chromium-molybdenum) and a pixel electrode (source electrode) 105 madeof an indium tin oxide (ITO) film are disposed so as to partly overlapthe pattern of the semiconductor film 116. A protective insulating film108 made of silicon nitride is formed so as to cover all of thesemembers.

Further, as shown in FIG. 2C, the common electrode 103 connected to thecommon electrode line 120 is disposed on an organic protective film(overcoat layer) 112 via the through hole 118 formed through the gateinsulating film 107 and the protective insulating film 108.

Further, as shown in FIG. 2A, the common electrodes 103 extending fromthe common electrode line 120 via the through holes 118 are disposedtwo-dimensionally in one pixel region such that they face the pixelelectrode 105 of the pixel.

In this embodiment, the pixel electrodes 105 are disposed below theprotective insulating film 108 which is disposed below the organicprotective film 112, and the common electrodes 103 are disposed on theorganic protective film 112.

A region sandwiched by these plural pixel electrodes 105 and the commonelectrodes 103 constitutes one pixel.

Further, a liquid crystal alignment film 109 is formed on the surface ofthe active matrix substrate having the thus constructed unit pixelsdisposed thereon in a matrix shape, that is, the liquid crystalalignment film 109 is formed on the organic protective film 112 on whichthe common electrodes 103 are formed.

Meanwhile, as shown in FIG. 1, a color filter layer 111 is disposed onthe glass substrate 102 constituting an opposite substrate so as to bepartitioned into sections for the individual pixels by a light shieldingfilm (black matrix) 113. In addition, the color filter layer 111 and thelight shielding film 113 are covered with the organic protective film112 made of a transparent insulating material. Further, the liquidcrystal alignment film 109 is formed also on the organic protective film112 to constitute the color filter substrate.

To these liquid crystal alignment films 109, the liquid crystalalignment capability is imparted by irradiation with linearly polarizedultraviolet light obtained from a low-pressure mercury-xenon lamp as alight source using a pile polarizer obtained by laminating quartzplates.

The glass substrate 101 constituting the active matrix substrate and theglass substrate 102 constituting the color filter substrate are disposedso as to face each other with respect to the surfaces of the liquidcrystal alignment films 109, respectively, and a liquid crystal layer(liquid crystal composition layer) 110 b composed of liquid crystalmolecules 110 a is disposed between the glass substrates.

Further, the glass substrate 101 constituting the active matrixsubstrate and the glass substrate 102 constituting the color filtersubstrate have polarizing plates 114 formed on the outer surfacesthereof, respectively.

As described above, an active matrix type liquid crystal display deviceusing TFT 115 (TFT liquid crystal display device) is constructed. Inthis TFT liquid crystal display device, when no electric field isapplied, the liquid crystal molecules 110 a constituting the liquidcrystal composition layer 110 b are aligned substantially in parallel tothe surfaces of the glass substrates 101 and 102 disposed so as to faceeach other, and are homogeneously aligned in an initial alignmentdirection determined by the photo-alignment treatment.

When TFT 115 is turned on by applying a voltage to the scan line 104, anelectric field (applied voltage) 117 is applied to the liquid crystalcomposition layer 110 b due to the electric potential difference betweenthe pixel electrode 105 and the common electrode 103, and the liquidcrystal molecules 110 a constituting the liquid crystal compositionlayer 110 b turn in the direction of the electric field due to theinteraction between the dielectric anisotropy of the liquid crystalcomposition layer 110 b and the electric field. In this case, the lighttransmittance of the liquid crystal display device is changed by therefractive anisotropy of the liquid crystal composition layer 110 b andthe action of the polarizing plates 114, whereby display can be carriedout.

The organic protective film 112 may be made of a thermosetting resinsuch as an acrylic resin, an epoxy-acrylic resin, or a polyimide resinhaving excellent insulating property and transparency. The organicprotective film 112 may also be made of a photo-curable transparentresin or an inorganic material such as a polysiloxane resin. Further,the organic protective film 112 may serve also as the liquid crystalalignment film 109.

As described above, according to the present embodiment, it is possibleto impart uniform alignment to the entire display region without causinglocal alignment disturbance near the electrodes by adopting anon-contact photo-alignment treatment, not a rubbing alignment treatmentin which the ability to align liquid crystal molecules of the liquidcrystal alignment film 109 is imparted by direct rubbing with a buffcloth.

In general, it is known that, unlike the vertical electric field modetypified by the conventional TN mode, the IPS mode fundamentally doesnot require an interface tilt with respect to the surface of a substrateand as the interface tilt angle is smaller, the visual anglecharacteristic is higher. Also in a liquid crystal alignment film havingbeen subjected to a photo-alignment treatment, a small interface tiltangle is preferred, and the interface tilt angle of 1 degree or less isparticularly effective, since the thus-formed liquid crystal displaydevice can remarkably suppress the color and luminance changes dependingon the viewing angles.

Subsequently, a specific manufacturing method according to thisembodiment will be described. As the glass substrate 101 constitutingthe active matrix substrate and as the glass substrate 102 constitutingthe color filter substrate, a surface-polished glass substrate having athickness of 0.7 mm is used.

The TFT 115 to be formed on the glass substrate 101 is composed of thepixel electrode 105, signal line 106, scan line 104, and semiconductorfilm 116 made of amorphous silicon.

All of the scan line 104, common electrode line 120, and signal line 106were formed by patterning a chromium film, and the distance between thepixel electrode 105 and the common electrode 103 was set to 7 μm.Incidentally, in this embodiment, the common electrode 103 and the pixelelectrode 105 are made of an ITO film by forming transparent electrodesfor achieving a higher luminance characteristic. However, it is alsopossible to form the common electrode 103 and the pixel electrode 105 ofa chromium film which has low resistivity and facilitates patterning inplace of an ITO film.

The gate insulating film 107 and the protective insulating film 108 weremade of silicon nitride and the thickness of each of these films was setto 0.3 μm. An acrylic resin was applied thereto and treated by heatingto 220° C. for 1 hour, thereby forming an organic protective film 112which was transparent and had an insulating property.

Subsequently, as shown in FIG. 2C, the through hole 118 was formed downto the common electrode line 120 by photolithography and etchingtreatment, and the common electrode 103 connected to the commonelectrode line 120 was formed by patterning.

As a result, in a unit pixel (one pixel), as shown in FIG. 2A, the pixelelectrode 105 was disposed among the three common electrodes 103, and anactive matrix substrate having 1024×3×768 pixels, namely, pixelscomposed of 1024×3 (corresponding to R, G, and B) signal lines 106 and768 scan lines 104 was formed.

EXAMPLE 1

A film containing a compound represented by the following chemicalformula (2) was formed on a glass substrate having a comb-like electrodeITO pattern by spin-coating a liquid crystal alignment agent varnishcontaining a precursor of the compound represented by the followingchemical formula (2), followed by heating to 230° C.

The film containing the compound represented by the above chemicalformula (2) formed on the substrate having a comb-like electrode wasirradiated with polarized ultraviolet light at a wavelength of 257 nmsuch that the illumination intensity and time were adjusted so as togive an exposure dose of 1 J/cm². When the polarized dichroic ratio of acarbonyl group at a wavelength of 1710 cm⁻¹ was determined from thedifference spectrum of the polarized spectrum, it was found to be 0.10.

On the other hand, after the irradiation with polarized ultravioletlight, the surface of the film having undergone the irradiation withultraviolet light was heated to 230° C. for 10 minutes, and then, thepolarized dichroic ratio was determined in the same manner withpolarized FT-IR and found to be 0.14.

The increase in the polarized dichroic ratio by the heating to 230° C.is due to the evaporation of the photodegraded product (low-molecularweight component).

Further, the above-mentioned two substrates were bonded to each otherusing a sealing material containing beads, a commercially availableliquid crystal was introduced into the space between the two substrates,and a completely sealed unit cell was prepared.

When the anchoring strength between the liquid crystal alignment filmand the liquid crystal was measured using this unit cell, it was foundthat the anchoring effect in the case of using the substrate obtained byheating to 230° C. for 10 minutes after the irradiation with polarizedultraviolet light was about 1.4 times higher than that in the case ofusing the substrate obtained without heating.

As a comparative example, a liquid crystal alignment film was preparedby subjecting a film formed by the same coating step as described aboveto a photo-alignment treatment through irradiation with polarizedultraviolet light at 200° C. Then, the polarized dichroic ratio wasdetermined for the thus obtained liquid crystal alignment film and foundto be 0.1.

The anchoring strength in the case of using the liquid crystal alignmentfilm of the comparative example was found to be about half of that inthe case of using the substrate obtained by heating to 230° C. for 10minutes after the irradiation with polarized ultraviolet light.

EXAMPLE 2

A film containing a compound represented by the following chemicalformula (1) was formed on a glass substrate having a comb-like electrodeITO pattern by spin-coating a liquid crystal alignment agent varnishcontaining a precursor of the compound represented by the followingchemical formula (1), followed by heating to 230° C. R in the chemicalformula (1) is an electron-donating group.

The film formed on the substrate having a comb-like electrode wasirradiated with polarized ultraviolet light at a wavelength of 257 nmsuch that the illumination intensity and time were adjusted so as togive an exposure dose of 1 J/cm². When the polarized dichroic ratio of acarbonyl group at a wavelength of 1710 cm⁻¹ was determined from thedifference spectrum of the polarized spectrum, it was found to be 0.05.

On the other hand, after the irradiation with polarized ultravioletlight, the surface of the film having undergone the irradiation withultraviolet light was heated to 230° C. for 10 minutes, and then, thepolarized dichroic ratio was determined in the same manner withpolarized FT-IR and found to be 0.23.

The increase in the polarized dichroic ratio by the heating to 230° C.is due to the evaporation of the photodegraded product (low-molecularweight component).

Further, the above-mentioned two substrates were bonded to each otherusing a sealing material containing beads, a commercially availableliquid crystal was introduced into the space between the two substrates,and a completely sealed unit cell was prepared.

When the anchoring strength between the liquid crystal alignment filmand the liquid crystal was measured using this unit cell, it was foundthat the anchoring effect in the case of using the substrate obtained byheating to 230° C. for 10 minutes after the irradiation with polarizedultraviolet light was about 4 times higher than that in the case ofusing the substrate obtained without heating.

As a comparative example, a liquid crystal alignment film was preparedby subjecting a liquid crystal alignment film formed by the same coatingstep as described above to a photo-alignment treatment throughirradiation with polarized ultraviolet light at 200° C. Then, thepolarized dichroic ratio was determined for the thus obtained liquidcrystal alignment film and found to be 0.1.

The anchoring strength in the case of using the liquid crystal alignmentfilm of the comparative example was found to be about half of that inthe case of using the substrate obtained by heating to 230° C. for 10minutes after the irradiation with polarized ultraviolet light.

While there have been described what are at present considered to becertain embodiments of the invention, it will be understood that variousmodifications may be made thereto, and it is intended that the appendedclaim cover all such modifications as fall within the true spirit andscope of the invention.

What is claimed is:
 1. A manufacturing method of a liquid crystaldisplay device, which is a manufacturing method of a liquid crystaldisplay device including a liquid crystal alignment film to which thealignment regulating force is imparted by a photo-alignment treatment,comprising: a film forming step of forming a film containing a polymerwhose main chain is cleaved by irradiation with light, wherein thepolymer is a compound represented by the following chemical formula (1):

wherein R represents a divalent electron-donating organic group selectedfrom an amino group (—NH—) and an alkoxy group (—OC_(n)H_(2n)—); and nrepresents the number of repeating units of the polymer; aphoto-alignment step of imparting the alignment regulating force to thefilm formed in the film forming step by irradiation of the film withlight in an atmosphere of a temperature lower than 100° C.; and aremoving step of removing a low-molecular weight component generated bycleaving the main chain of the polymer through the light irradiationafter the light irradiation.
 2. The manufacturing method of a liquidcrystal display device according to claim 1, wherein in the removingstep, the low-molecular weight component is removed by heating the filmafter the light irradiation.
 3. The manufacturing method of a liquidcrystal display device according to claim 2, wherein the heating in theremoving step is performed under a reduced pressure atmosphere.
 4. Themanufacturing method of a liquid crystal display device according toclaim 1, wherein in the removing step, the low-molecular weightcomponent is removed by washing the film after the light irradiation. 5.The manufacturing method of a liquid crystal display device according toclaim 4, wherein the washing in the removing step is performed using asolution containing a water-soluble organic solvent.
 6. Themanufacturing method of a liquid crystal display device according toclaim 4, wherein the washing in the removing step is performed using asolution containing a surfactant.
 7. A liquid crystal display devicemanufactured by the manufacturing method according to any one of claims2 to
 6. 8. A liquid crystal display device manufactured by themanufacturing method according to claim
 1. 9. The manufacturing methodof a liquid crystal display device according to any one of claims 2 to1, wherein photo-alignment step of imparting the alignment regulatingforce to the film formed in the film forming step by irradiation of thefilm with light in an atmosphere of a temperature lower than 75° C. 10.The manufacturing method of a liquid crystal display device according toany one of claims 2 to 1, wherein photo-alignment step of imparting thealignment regulating force to the film formed in the film forming stepby irradiation of the film with light in an atmosphere of a temperaturelower than 50° C.